On of the school compounds was "repeatedly attacked," adding that it may be the deadliest attack on a school since the war began more than five years ago.

A boy inspects a damaged site after shelling in the rebel held town of Hass, south of Idlib province, Syria October 26, 2016. (Source: Reuters)

Air strikes that hit a school in Syria’s rebel-held Idlib province killed 22 children and six teachers, the UN children’s agency UNICEF has said. “This is a tragedy. It is an outrage. And if deliberate, it is a war crime,” UNICEF director Anthony Lake said on Wednesday.

The Syrian Observatory for Human Rights said “warplanes — either Russia or Syrian — carried out six strikes” in the village of Hass, including on a school complex, killing 11 schoolchildren.

Lake said the school compound was “repeatedly attacked,” adding that it may be the deadliest attack on a school since the war began more than five years ago.

A photograph circulated on social media showed a child’s arm, seared off above the elbow, still clutching the strap of a dusty black rucksack.

“When will the world’s revulsion at such barbarity be matched by insistence that this must stop?” added the UNICEF director.